The invention relates to a deflection unit for colour cathode-ray tubes, which unit is also called a deflector and comprises a pair of vertical deflection coils and a pair of horizontal deflection coils in the form of a saddle, whose particular shape allows the coma errors to be minimized.
A cathode-ray tube designed to generate colour images generally comprises an electron gun emitting three coplanar electron beams, each beam being intended to excite bands of luminescent material of the corresponding colour (red, green or blue) on the tube""s screen.
The electron beams scan the tube""s screen under the influence of the deflection fields created by the horizontal and vertical deflection coils of the deflector which is fixed to the neck of the tube. A ring of ferromagnetic material conventionally surrounds the deflection coils so as to concentrate the deflection fields in the appropriate region.
The three beams generated by the electron gun must always converge on the tube""s screen or else suffer the introduction of a so-called convergence error which, in particular, falsifies the rendition of the colours. In order to achieve convergence of the three coplanar beams, it is known to use so-called self-converging astigmatic deflection fields; in a self-converging deflection coil, the intensity of the horizontal-deflection field then has a pincushion-shaped distribution and that of the vertical deflection field has a barrel-shaped distribution.
Coma is an aberration which affects the side beams coming from an electron gun having three beams in line, independently of the astigmatism of the deflection fields and of the curvature of the screen surface of the tube; these side beams enter the deflection region at a small angle with respect to the axis of the tube and undergo a deflection in addition to that of the axial beam. The coma is generally corrected by modifying the distribution of the deflection fields at the point where the beam enters the deflector so that the coma generated compensates for that produced by the field distribution necessary to obtain the desired astigmatism for self-convergence. Thus, with regard to the horizontal deflection field, the field at the rear of the deflector has the shape of a barrel and in the front part has the shape of a pincushion.
Field configurations like those described above may cause the appearance of aberrations called coma parabolas which are manifested in a rectangular test pattern displayed on the tube""s screen by an increasing shift of the green image with respect to the red/blue image as one approaches the corners of the test pattern. If the shift is towards the outside of the test pattern, the coma error is conventionally positive, while if it is towards the inside of the said test pattern the coma error is negative.
Simultaneous control, by means of a particular configuration of the conductors making up the deflection coils, of the coma, coma parabola, convergence and geometry errors has not hitherto been possible without adding additional components, such as metal pieces, arranged so as to cause local modification of the deflection fields for the purpose of correcting the coma errors, or permanent magnets for correcting the geometry defects. French Patent No. 2,757,678 describes a horizontal deflection coil configuration allowing correction of either vertical or horizontal coma parabola problems; however, since the horizontal and vertical coma parabolas are intimately linked variation-wise, the configuration described by the said application does not make it possible to correct both horizontal and vertical coma parabolas, particularly when the latter are of opposite sign, this being due to their opposing behaviour, correction of one causing degradation of the other.
Moreover, these problems of the geometry of the image, of coma and of convergence are connected with the planarity of the screen and with the size of the latter. Conventional cathode-ray tubes manufactured a few years ago and using a screen of spherical shape generally have a small radius of curvature. Since the current trend is moving towards screens of large radius of curvature, or else completely flat screens, with diagonals greater than 70 cm in length, it is becoming increasingly difficult to control the abovementioned problems solely by means of suitable fields generated by the deflection coils.
It is common practice to divide the deflection system into three successive action regions along the main axis of the tube: the rear region closest to the electron gun influences more particularly the coma, the intermediate region acts more particularly on the astigmatism of the deflection field and therefore on the convergence of the red and blue electron beams and, finally, the front region, lying closest to the tube""s screen, acts on the geometry of the image which will be formed on the tube""s screen.
The object of the present invention is to make it possible, by a particular arrangement of the winding wires of the vertical deflection coils, to generate deflection fields no longer requiring the use of additional correctors to minimize the coma parabola errors down to an acceptable level, without irreversibly modifying the other design parameters of the deflector, such as the convergence of the electron beams and the geometry of the image formed on the tube""s screen.
To do this, the electromagnetic deflection unit for cathode-ray tubes according to the invention comprises a pair of horizontal deflection coils and a pair of vertical deflection coils, the vertical deflection coils being in the shape of a saddle and comprising a front bundle lying on the screen side of the tube and a rear bundle lying on the electron-gun side, the said bundles being connected to each other by lateral conductor harnesses, the front and rear bundles and the lateral harnesses defining a window free of conductors, which deflection unit is characterized in that, in the region close to the rear bundle, at least 98% of the lateral harness conductors lie within an angular aperture "THgr"m of less than 80xc2x0.